To elucidate DNA trajectory in the p53-DNA complex in solution, we are using Iodine-125 radioprobing (in collaboration with I. Panyutin and R. Neumann, Clinical Center, NIH). This method is based on analysis of the DNA strand breaks produced by the decay of an electron-emitting radioisotope, Iodine-125, incorporated in the C5 position of cytosine. The weaker the DNA strand break, the larger the distance from the radioisotope to the cleavage site. The major advantage of radioprobing is its applicability for very large protein-DNA complexes. In particular, this method allows direct comparison of the conformations of DNA bound to the p53 core domain and to the wild type protein, the latter still being beyond the scope of conventional methods such as crystallography and NMR. Our results indicate that in a tetrameric complex with wt p53, the central region of the consensus 20-bp DNA fragment (YYYRRR) is bent into the minor groove (that is, consistent with our model and with the p53 tetramer binding to nucleosome). The detailed visualization of the DNA trajectory requires more radioprobing data. Recently, such data were obtained for several DNA sequences, including those of the p53 REs activating cell cycle arrest and apoptotic genes (CCA and Apo-genes). Currently, we are analyzing the radioprobing results, comparing the observed DNA strand breaks intensities with the sugar-iodine distances deduced from the p53-DNA co-crystal structures. To compare the chromatin context of the p53 sites associated with the CCA- and Apo-genes, we analyzed the sequence-dependent bending anisotropy of human genomic DNA containing p53 sites. We calculated rotational positioning patterns predicting that most of the CCA-sites are exposed on the nucleosomal surface. This is consistent with experimentally observed positioning of human nucleosomes. Remarkably, the sequence-dependent DNA anisotropy of both the p53 sites and flanking DNA work in concert producing strong positioning signals. By contrast, both the predicted and observed rotational settings of the Apo-sites in nucleosomes suggest that many of these sites are buried inside, thus preventing immediate p53 recognition and delaying gene induction. We also measured the p53 binding to its cognate sites embedded in the in strongly positioned '601' nucleosome. Our data suggest that the p53 affinity to DNA strongly correlates with the rotational positioning of its site in nucleosome, in agreement with the computational analysis described above. The exposed configurations of the p53 sites in nucleosome (like CCA-sites) demonstrate significantly stronger affinity to p53 compared to the buried configurations (similar to the Apo-sites). Thus, the difference in nucleosomal organization of the two sets of p53 response elements appears to be a key factor affecting the strength of p53-DNA binding and kinetics of induction of the p53 target genes. Our model differs from the earlier concept connecting the selective activation of the CCA- and Apo-genes to the binding affinities of their REs to p53. Instead, we emphasize a direct correlation between the selection of p53-induced tumor suppression pathway (apoptosis versus cell cycle arrest) and structural organization of the corresponding p53-binding sites in chromatin. We add new dimensions to the existing paradigm, the relative positioning and chromatin environment of the p53 REs. Our scheme not only explains the above cases but also provides a new insight into the cellular mechanisms of activation of hundreds of genes by p53.